Image formation with gradation value of boundary pixels corrected using cumulative sum of grataion values of pixels in significant image portion

ABSTRACT

An image forming apparatus includes a reading unit for sequentially reading, from image data, pieces of pixel data of target pixels along a predetermined direction, a cumulative sum calculation unit for calculating a cumulative sum of gradation values represented by pieces of pixel data of target pixels in a significant image portion read by the reading unit, a determination unit for determining whether the target pixel read by the reading unit is located at a boundary between the significant image portion and a background portion that is a non-significant image portion, and a correction unit for correcting, if the determination unit determines that the target pixel is located at the boundary, a gradation value represented by pixel data of the target pixel by adding a correction amount based on the cumulative sum up to the target pixel calculated by the cumulative sum calculation unit to the gradation value.

BACKGROUND

Field of Art

The present disclosure relates to a technique for forming an image on arecording medium using an electrophotographic method.

Description of the Related Art

Electrophotographic image forming apparatuses are known. Such an imageforming apparatus forms an electrostatic latent image by emitting lightonto a photosensitive member on the basis of image data, applies adeveloper to the electrostatic latent image (performs development),transfers and fixes the image to a recording medium such as a recordingsheet, and outputs the recording medium. Representative examples of suchan image forming apparatus include a laser beam printer and an LEDprinter. In such an image forming apparatus, at the time of the emissionof a light beam or the formation of an electrostatic latent image, imagedegradation occurs. The contrast of a latent image potential formed on aphotosensitive member decreases at that time. The relationship between alatent image potential on a photosensitive member and the amount oftoner applied as a developer at the time of development is nonlinear,and is affected by the decrease in the contrast of a latent imagepotential. In the case of image data including a thin character or athin line, the contrast of a latent image potential easily decreases andthe thinning or detail loss of a line occurs at the time of formation ofan image. The reproducibility of an output image therefore decreases.

Japanese Patent Laid-Open No. 2000-36912 or 2009-105943 discloses atechnique for controlling the line width of input image data to improvethe reproducibility of an output image. More specifically, a line widthin image data is detected by referring to the image data of a portionaround the edge of a thin line and the gradation value of the edge ischanged using the amount of correction corresponding to the detectedline width, so that the thin-line reproducibility of an output image isimproved. With the technique disclosed in Japanese Patent Laid-Open No.2000-36912 or 2009-105943, the amount of correction is changed inaccordance with a line width in image data. It is therefore consideredto be possible to faithfully reproduce a thin line on an output image onthe basis of the image data.

However, the technique disclosed in Japanese Patent Laid-Open. No.2000-36912 or 2009-105943, in which the image data of a portion aroundthe edge of a thin line is referred to for the detection of the width ofthe thin line, requires a large hardware circuit size and a highcalculation cost.

SUMMARY

An embodiment of the present invention provides an image formingapparatus for forming an image by performing scanning exposure upon aphotosensitive member on the basis of a gradation value represented bypixel data. The image forming apparatus includes a reading unitconfigured to sequentially read, from image data, pieces of pixel dataof target pixels along a predetermined direction, a cumulative sumcalculation unit configured to calculate a cumulative sum of gradationvalues represented by pieces of pixel data of target pixels in asignificant image portion read by the reading unit, a determination unitconfigured to determine whether the target pixel read by the readingunit is located at a boundary between the significant image portion anda background portion that is a non-significant image portion, and acorrection unit configured to, in a case where the determination unitdetermines that the target pixel is located at the boundary, correct agradation value represented by pixel data of the target pixel by addinga correction amount based on the cumulative sum up to the target pixelcalculated by the cumulative sum calculation unit to the gradationvalue.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C are diagrams describing the relationship between adata image width and a developed image width.

FIG. 2 is a block diagram illustrating the configuration of an imageforming apparatus according to a first embodiment of the presentinvention.

FIG. 3 is a block diagram illustrating the configuration of a processingunit according to the first embodiment.

FIG. 4 is a diagram illustrating a process according to the firstembodiment.

FIGS. 5A and 5B are diagrams illustrating exemplary processing accordingto the first embodiment.

FIG. 6 is a diagram illustrating an example of a correction amount tableaccording to the first embodiment.

FIG. 7 is a block diagram illustrating the configuration of a processingunit according to a second embodiment of the present invention.

FIG. 8 is a diagram illustrating a process according to the secondembodiment.

FIGS. 9A and 9B are diagrams illustrating exemplary processing accordingto the second embodiment.

FIGS. 10A and 10B are diagrams illustrating examples of a correctionamount table according to the second embodiment.

FIG. 11 is a block diagram illustrating the configuration of aprocessing unit according to a third embodiment of the presentinvention.

FIG. 12 is a diagram describing the configuration of a pixel accordingto the third embodiment.

FIG. 13 is a diagram illustrating exemplary processing according to thethird embodiment.

FIG. 14 is a diagram illustrating an example of a correction amounttable according to the third embodiment.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will be described in detail belowwith reference to the accompanying drawings.

First Embodiment

<Relationship Between Data Image Width and Developed Image Width>

In an electrophotographic image forming apparatus, as a line width inimage data decreases, an electrostatic latent image on a photosensitivemember becomes shallower and the thin-line reproducibility of an outputimage decreases. The relationship between an image width in image dataand an image width after development will be described below withreference to FIGS. 1A, 1B, and 1C by taking, as an example, awhite-on-black line susceptible to thinning or a detail loss because ofthe degradation of an electrostatic latent image.

FIG. 1A is a diagram illustrating exemplary pieces of image data ofhollow lines (horizontal lines on which scanning exposure is performedwith laser light) having a plurality of image widths. Referring to FIG.1A, I1, I2, I3, and I4 represent widths of one pixel, two pixels, threepixels, and fourth pixels, respectively. In this example, a printerengine with 600 dpi is used. The size of a single pixel is thereforeequivalent to one dot at 600 dpi (=2.54 cm/600≈0.0423=42.3 μm). In theimage data of a white-on-black line, a pixel represented by white is apart of a significant image and a pixel represented by black is anon-significant around portion (background portion). A significant imageportion and a non-significant image portion are hereinafter referred toas an image portion and a ground portion, respectively.

FIG. 1B is a diagram illustrating the potential distribution of anelectrostatic latent image that is formed on a photosensitive memberafter exposure on the basis of each piece of image data illustrated inFIG. 1A. Referring to FIG. 1B, E1, E2, E3, and E4 represent thepotential distributions of electrostatic latent images corresponding toI1, I2, I3, and I4 in. FIG. 1. As is apparent from FIG. 1B, the narrowerthe width of a white portion, the lower the contrast of a latent imagepotential (the difference between the maximum value and the minimumvalue) because of image degradation at the time of the emission of lightbeam or the formation of an electrostatic latent image (the contrast ofa latent image potential: E1<E2<E3<E4). FIG. 1C is a diagramillustrating exemplary development line widths of toner images formedfrom the electrostatic latent images using a development biasrepresented by a dotted line in FIG. 1B. Referring to FIG. 1C, W1, W2,W3, and W4 represent development line widths corresponding to E1, E2,E3, and E4 in FIG. 1B. A dotted line in FIG. 1C represents a target linewidth (that is linear with respect to a data line width). The lower thelatent image contrast, the larger the amount of toner transferred to awhite portion. Accordingly, because of a toner fog or toner scatteredfrom surrounding areas, the width of the hollow line decreases (becomesblack) and goes away from the target line width (the difference from thetarget line width W1>W2>W3>W4). Another exemplary case other than theimage data illustrated in FIG. 1A can be considered. For example, in thecase of a pixel having a plurality of gradation levels, it is possibleto control a line width by unit smaller than a single pixel bymodulating the ON and OFF lengths of light beam in accordance with agradation level (pulse width modulation). In this case, the relationshipbetween an image data line width and a development line width is asrepresented by a solid line in FIG. 1C.

Descriptions have been made by taking only a hollow line as an example.In the case of a black line, when a latent image contrast decreases, thethinning or detail loss of a line may also occur because of thereduction in a toner adhesion amount or the scattering of toner tosurroundings.

In the first embodiment, in order to suppress the occurrence of thethinning or detail loss of a thin line to improve the reproducibility ofa developed image, correction processing is performed upon image data.

<System Configuration>

FIG. 2 is a block diagram of an image forming apparatus 2 according tothe first embodiment. An image processing apparatus 1 that is a supplysource of recording target image data is connected to the image formingapparatus 2. The image processing apparatus 1 and the image formingapparatus 2 may communicate with each other using any communicationmethod such as a wireless communication method (WiFi or Bluetooth®) or awired communication method (USE interface or Ethernet interface). Inthis embodiment, the image processing apparatus 1 is an informationprocessing apparatus such as common personal computer. An applicationfor a print target image data and a printer driver for generating printdata for the image forming apparatus 2 are installed in advance into theimage processing apparatus 1.

The reduction in the reproducibility of an output image due todegradation in an electrostatic latent image depends on the enginedesign of the image forming apparatus 2, for example, the spot diameterof light beam or the latent image formation characteristics of aphotosensitive drum. In order to absorb the difference inreproducibility due to engine design, is desired that line widthcorrection processing be performed on an engine side. In the firstembodiment, line width correction processing is therefore performed onan engine side on the basis of information about image data transmittedfrom a printer driver side. On the engine side, a shared memory used asan information processing work area or a temporary storage area isprovided independently of a shared memory on the printer driver side.

The first embodiment provides an image forming apparatus and an imageforming program capable of more easily improving thin-linereproducibility as compared with those not having this configuration.

<Image Forming Apparatus>

The image forming apparatus 2 includes a central processing unit (CPU)201 for performing overall control processing, a hard disk drive (HDD)202 for storing programs for line width correction processing and pulsewidth modulation processing which are performed by the CPU 201, and aRandom Access Memory (RAM) 203 used as a work area at the time of theexecution of a program. The CPU 201 executes a program stored in the RAM203, performs line width correction processing and pulse widthmodulation processing upon image data stored in the RAM 203, and outputsthe processed image data to a driving unit 204.

In the line width correction processing, the values of pixels(=gradation information) included in image data are sequentially readout. On the basis of the read gradation information of a pixel(hereinafter referred to as a “target pixel”), cumulative gradationvalue that s the cumulative sum of pieces of gradation information of acontinuous image portion up to the target pixel is calculated. In a casewhere it is determined that a target pixel is located at the boundarybetween the image portion and a ground portion, the ground portion ischanged to the image portion on the basis of the amount of image widthcorrection based on the cumulative gradation value. That is, the widthof the image portion is increased. The line width correction processingwill be described in detail later.

The pulse width modulation processing is processing for expressing anintermediate density by modulating the ON and OFF lengths of light onthe basis of the gradation value of image data. More specifically, apulse signal having a width based on the gradation value of image datais generated. In accordance with this pulse, the ON and OFF of lightbeam are controlled. In the case of an image forming apparatus in whichtoner is applied to a portion exposed to light, the larger the gradationvalue of a black pixel, the longer the ON length of light (the largerthe gradation value of a white pixel, the longer the OFF length oflight). In this embodiment, control processing for shifting a pulseforward in unit pixels and control processing for shifting a pulserearward in unit pixels are switched. By shifting the position of apulse to couple the pulse to a surrounding pulse, an electrostaticlatent image is stabilized. In a case where the image processingapparatus 1 creates shift control information indicating which directioneach pixel is coupled to, the CPU 201 receives shift control informationalong with image data and stores them in the RAM 203. In this case, inthe pulse width modulation processing, a pulse signal is generated onthe basis of the gradation value of image data and the shift controlinformation.

An image forming unit includes an exposure unit 208, a photosensitivedrum 212 that is an image bearing member, a charger 216, and a developer220.

Electrophotographic Image Formation will be Described Below.

(1) Charging

The charger 216 charges the surface of the rotating photosensitive drum212 at a predetermined potential. The charging potential of the charger216 is measured by an electrometer 241 facing the photosensitive drum212 and is controlled so that a desired voltage is obtained.

(2) Exposure

The driving unit 204 modulates image data supplied from the CPU 201 intoan exposure signal by exposure signal generation processing and drivesthe exposure unit 208 in accordance with the exposure signal. Theexposure unit 208 performs scanning exposure on the rotatingphotosensitive drum 212 to form an electrostatic latent image on thephotosensitive drum 212.

(3) Development

The developer 220 applies toner to the electrostatic latent image formedon the rotating the photosensitive drum 212 to generate a toner image.

(4) Transfer

A voltage is applied to a transfer roller 230 at a transfer nip where arecording medium (recording sheet) 231 and the photosensitive drum 212are brought into contact with each other to transfer the toner image tothe recording medium 231.

(5) Fixing

The recording medium including the toner image on its surface istransferred to a fixing unit 232. The heated fixing unit 232 appliesheat and pressure to the recording medium and the toner image on therecording medium to fuse the toner image into place on the recordingmedium.

(6) Sheet Discharge

The recording medium including an output image on its surface isdischarged from the image forming apparatus 2 via the fixing unit 232.Electrophotographic image formation ends in the image forming apparatus2.

<Details of Line Width Correction Processing>

Next, the line width correction processing performed by the imageforming apparatus 2 according to an embodiment of the present inventionwill be described in more details. FIG. 3 is a functional block diagramof a line width correction processor realized when a program related toimage formation is loaded from the HDD 202 into the RAM 203 and isexecuted by the CPU 201. Each unit is a procedure, a function, or asubroutine of the CPU 201, but may be realized with a piece of hardware.

A line width correction processor 21 performs line width correctionprocessing upon image data transmitted from a printer driver in theimage processing apparatus 1. The line width correction processor 21includes an image data storage unit 2101, a cumulative gradation valueacquisition unit 2102, an edge determination unit 2103, a correctionunit 2104, and a correction amount storage unit 2105. A pulse widthmodulation processor 22 modulates the ON and OFF lengths of light on thebasis of the gradation value of image data to express an intermediatedensity. The pulse width modulation processor 22 include, for example, atriangular wave generation circuit for generating an analog triangularwave synchronized with a pixel clock, a D/A circuit for converting thegradation value of image data into an analog signal (voltage signal),and a comparator for comparing a triangular wave generated by thetriangular wave generation circuit and an analog signal representing thegradation value of pixel with each other. A signal output from thecomparator pulse-width modulation. (PWM) signal having a widthcorresponding to the gradation value of a pixel.

It is assumed that image data transmitted from the image processingapparatus 1 is expressed with 8 bits per pixel (256 gradation levels).

The CPU 201 acquires image data in a scanning direction (predetermineddirection) of light beam output from the image forming apparatus 2 inunits of lines and stores them in the image data storage unit 2101 inthe RAM 203. The cumulative gradation value acquisition unit 2102sequentially reads pieces of pixel data stored in the image data storageunit 2101, acquires the cumulative sum of gradation values of pixels inthe image data, and stores the cumulative sum as a cumulative gradationvalue. In the first embodiment, in order to improve the reproducibilityof a hollow line where thinning or a detail loss easily occurs becauseof the image degradation of an electrostatic latent image, line widthcorrection processing is performed focusing on the gradation level of awhite portion. That is, calculation is performed on the assumption thatthe gradation value of a white pixel (having the minimum density level)of image data is 255 and the gradation value of a black pixel (havingthe upper limit of a density level) of image data is 0. In a case whereimage data in which the gradation value of a white pixel is 0 and thegradation value of a black pixel is 255 is input from the imageprocessing apparatus 1, gradation values (0 to 255) are reversed. Theedge determination unit 2103 detects the boundary between an imageportion and a ground portion of image data. The correction amountstorage unit 2105 stores the amount of gradation correction based on acumulative gradation value. The correction unit 2104 corrects thegradation value of a target pixel determined to be an edge by the edgedetermination unit 2103 on the basis of the amount of correction storedin the correction amount storage unit 2105. The corrected image data isoutput to the pulse width modulation processor 22.

<Line Width Correction Process>

FIG. 4 is a flowchart illustrating a process performed by the line widthcorrection processor 21. When an image formation start instruction ismade, the CPU 201 receives image data in units of lines from a printerdriver and transmits the image data to the image data storage unit 2101.In step S101, the CPU 201 performs line width correction processingwhile sequentially reading out pixels included in image data of a targetline.

FIG. 5A is a diagram illustrating exemplary image data including hollowoblique lines with different widths (L11=4-pixel width, L12=2-pixelwidth, and L13=1-pixel width). A hatched area may have already beensubjected to smoothing processing. In smoothing processing, a whitepixel (having the minimum density level) or a black pixel (having themaximum density level) is replaced with a halftone pixel to suppress theoccurrence of jaggy (a phenomenon where an edge is not smooth and showsjaggedness). This smoothing processing is performed in a printer driver,but may be performed in an image forming apparatus.

Referring to FIG. 5B, a reference numeral 501 represents a series ofpixels in a line L illustrated in. FIG. 5A as exemplary image data of aprocessing line read in step S101. In the first embodiment, line widthcorrection processing is performed while sequentially reading targetpixels from a leftmost pixel P1 to a pixel P16. A numerical value ineach pixel in the image data 501 represents a white gradation level. Inthis embodiment, the gradation level of a white pixel (having theminimum density level) is set to 255, and the gradation level of a blackpixel (having the maximum density level) is set to 0.

In step S102, the cumulative gradation value acquisition unit 2102calculates a cumulative gradation value that is the cumulative sum ofgradation values of pixels in a target pixel update direction. A dashedframe 502 in FIG. 5B represents a cumulative gradation value calculatedand stored by the cumulative gradation value acquisition unit 2102 wheneach pixel (P1 to P16) becomes a target pixel. In a case where thegradation value of a target pixel is greater than or equal to 1 (thereis a significant image component), the cumulative gradation valueacquisition unit 2102 adds the gradation value of the target pixel to acumulative gradation value at a preceding pixel to calculate acumulative gradation value at the target pixel. In a case where thegradation value of a target pixel is 0, the cumulative gradation valueacquisition unit 2102 resets a cumulative gradation value to 0. Acumulative gradation value is also reset in a case where a target pixelis the leftmost pixel (accumulation start pixel) in a line. For example,when a target pixel is P1, a cumulative gradation value is reset to 0because the gradation value of P1 is 0 (or P1 is the leftmost pixel).When a target pixel is P2, the gradation value of P2 is added to acumulative gradation value because the gradation value of P2 is greaterthan or equal to 1. In this example, the gradation value of 153 of thetarget pixel is added to the cumulative gradation value of 0 at thepreceding pixel, so that a cumulative gradation value at the targetpixel becomes 153. When a target pixel is P3, the gradation value of P3is added to the cumulative gradation value because the gradation valueof P3 is greater than or equal to 1. In this example, the gradationvalue of 255 of the target pixel is added to the cumulative gradationvalue of 153 at the preceding pixel, so that a cumulative gradationvalue at the target pixel becomes 408. Thus, at pixels P4 to P16,processing is similarly performed.

In this embodiment, when a cumulative gradation value is greater than orequal to 1024, a signal value of 1024 is output to limit the number ofbits of a signal. The reason for this is that it is unnecessary toperform line width correction processing upon an image including a heavyline where a cumulative gradation value of 1024 is continuous.

In step 3103, the edge determination unit 2103 determines whether atarget pixel is the edge of an image portion. A dashed frame 503 in FIG.5B represents whether the edge determination unit has determined thateach pixel (P1 to P16) is an edge when becoming a target pixel (edge, 1and non-edge: 0). In the first embodiment, when the gradation value of atarget pixel is greater than or equal to 1 (the target pixel includes asignificant image component) and the gradation value of a pixel adjacentto the target pixel (hereinafter referred to as an adjacent pixel) is 0(the pixel does not include a significant image component), the targetpixel is determined to be an image edge. For example, when a targetpixel is P6, P11, or P15, the target pixel is determined to an edge(“1”) since the gradation value of the target pixel is greater than orequal to 1 and the gradation value of a right-hand pixel is 0. On theother hand, when a target pixel is another pixel, the pixel isdetermined to be a non-edge (“0”) since the gradation value of thetarget pixel is 0 or the gradation value of the adjacent pixel isgreater than or equal to 1. In a case where the target pixel is an imageedge (S103: YES), the process proceeds to step S104. On the other hand,in a case where the target pixel is not an image edge (S103: NO), theprocess proceeds to step S106.

In step S104, the correction unit 2104 refers to a correction amounttable in the correction amount storage unit 2105 to acquire a correctionamount based on the cumulative gradation value. FIG. 6 is a diagramillustrating an example of a correction amount table stored in thecorrection amount storage unit 2105. This table stores gradationcorrection amounts corresponding to the cumulative gradation values of 1to 1024. The larger the cumulative gradation value, the smaller theamount of correction. Referring back to FIG. 5B, a dashed frame 504represents the amount of correction acquired from a correction amounttable in the correction amount storage unit 2105 on the basis of acumulative gradation value at a pixel determined to be an edge. Since acumulative gradation value at the pixel P6 is 1020, the amount ofcorrection becomes 0. Since a cumulative gradation value at the pixelP11 is 510, the amount of correction becomes 32.

In step S105, the correction unit 2104 adds the amount of correctionacquired in step S104 to the target pixel. In a case where the gradationvalue of the target pixel acquired after the addition of the amount ofgradation correction in step S105 is greater than the maximum gradationlevel of 255 that can be expressed at the target pixel, the excessgradation values are distributed to an adjacent pixel. Image data 505 inFIG. 5B represents a result of the addition of the gradation correctionamount in the dashed frame 504 to the image data 501 in step S105. In acase where a target pixel is P6, P11, or P15, a result of the additionof the correction amount in the dashed frame 504 to the gradation valuein the image data 501 is output. At that time, the larger the cumulativegradation value, the smaller the amount of correction. At the end ofimage data with a large pixel width, for example, at the pixel P6 at theedge of the hollow oblique line L11 with a four-pixel width, since acumulative gradation value is large, the amount of correction is small.On the other hand, at the end of image data with a small pixel width,for example, at the pixel P15 at the edge of the hollow oblique line L13with a 1-pixel width, since a cumulative gradation value is small, theamount of correction is large. As a result, it is possible toappropriately improve the reproducibility of an output image inaccordance with the line width of image data.

Using the pulse width modulation processing and a pulse signal shiftcontrol in combination, it is possible to more accurately adjust a linewidth. The reason for this is that even in a case where only a part of apixel is drawn, a line obtained by coupling pixels can be reproduced byshifting a pulse signal forward and rearward at the pixel. Thus, it isdesired that a pulse signal generated at an adjacent pixel to which acorrection value has been distributed from a target pixel, be subjectedto shift control to be coupled to a pulse generated at the target pixel.In the case of an engine that expresses a white pixel using a light beamOFF signal, in order to couple a light beam OFF signal at an adjacentpixel to a light beam OFF signal at a target pixel (in order to shift alight beam ON signal rearward), shift control information at theadjacent pixel is rewritten.

In step S106, the target pixel is shifted to the next pixel, forexample, in the right-hand direction pixel. In step S107, it isdetermined whether all pixels included in the image data acquired instep S101 have already been processed. In a case where it is determinedthat all pixels have yet to be processed, the process from step S102 tostep S106 is repeated. In a case where it is determined that all pixelshave already been processed, the process ends.

In the first embodiment, line width correction processing is performedat a target pixel in image data on the basis of the gradation value ofthe target pixel and the gradation value of an adjacent pixel. As aresult, it is possible to appropriately improve the reproducibility ofan output image with a small reference data range and the small amountof computation.

In the first embodiment, processing is performed on the basis of thegradation value of each pixel. Accordingly, even in the case of halftoneimage data that has been subjected to smoothing processing, it ispossible to perform line width correction processing upon the image databy appropriately detecting the image width and edge of the image data.

In the first embodiment, line width correction processing is performedin a direction parallel to line-by-line image data. However, line widthcorrection processing can also be performed in a direction orthogonal toline-by-line image data. In this case, the CPU 201 reads out two piecesof line-by-line image data and stores them in the image data storageunit 2101. The cumulative gradation value acquisition unit acquires, atthe position of each pixel in a line direction, a cumulative gradationvalue in a direction orthogonal to the line direction. One of the twopieces of line-by-line image data stored in the image data storage unit2101 is set as a target pixel line, and the other one of them is set asan adjacent pixel line. Line width correction processing is sequentiallyperformed in a line direction and a direction orthogonal to the linedirection while updating the two pieces of line-by-line image data oneby one in the image data storage unit 2101. At that time, since pulsewidth modulation processing and control processing for shifting a pulsesignal cannot be performed in combination in the direction orthogonal tothe line direction, the same effect cannot be obtained even if the sameamount of line width correction as that in the line direction is used inthe orthogonal direction. It is desired that the amount of line widthcorrection be adjusted in each direction and be stored in the correctionamount storage unit.

In the first embodiment, in order to improve the reproducibility of ahollow thin line where thinning or a detail loss easily occurs becauseof blurring of an electrostatic latent image, a cumulative gradationvalue is acquired focusing on a white gradation level. However, there isa case where the reproducibility of a black thin line has a higherpriority depending on an engine or image design. In such a case, linewidth correction processing may be performed using a cumulativegradation value acquired focusing on a black gradation level. That is,the gradation level of a white pixel (having the minimum density level)in image data is set to 0 and the gradation level of a black pixel(having the maximum density level) is set to 255 so that a white pixelbecomes a ground portion and a black pixel becomes an image portion. Thecalculation of a cumulative value of gradation levels of black pixelsand edge determination processing are then performed. As a result, thereproducibility of a black thin line can be improved.

In this embodiment, the correction amount storage unit 2105 stores acorrection table. The correction table does not necessarily have to bestored, and the amount of correction may be calculated using thefunction of a curve representing the relationship between a cumulativegradation value and a correction amount illustrated in FIG. 6.

Second Embodiment

In the first embodiment, a method of improving the reproducibility of anoutput image using a cumulative gradation value acquired focusing on thegradation level of an image portion has been described. In the secondembodiment, a method of more appropriately improving the reproducibilityof an output image using a cumulative gradation value acquired focusingon both the gradation level of an image portion and the gradation levelof a ground portion will be described. In this embodiment, the samereference numerals are used to identify operations and functions alreadydescribed in the first embodiment, and the description thereof will beomitted as appropriate.

FIG. 7 is a block diagram of the line width correction processor 21realized by the CPU 201 according to the second embodiment. The linewidth correction processor 21 includes the image data storage unit 2101,the cumulative gradation value acquisition unit 2102, a aroundcumulative gradation value acquisition unit 2112, the edge determinationunit 2103, a ground edge determination unit 2113, the correction unit2104, and the correction amount storage unit 2105.

The cumulative gradation value acquisition unit 2102 calculates andstores a cumulative gradation value that is the cumulative sum ofgradation values of pixels focusing on the gradation level of an imageportion in image data. In the second embodiment, calculation isperformed under the assumption that the gradation level of a white pixel(having the minimum density level) in image data is 255 and thegradation level of a black pixel (having the maximum density level) inimage data is 0. The ground cumulative gradation value acquisition unit2112 calculates and stores a ground cumulative gradation value that isthe cumulative sum of differences from the maximum gradation level of255 at pixels focusing on the gradation level of a ground portion inimage data. That is, calculation is performed under the assumption thatthe gradation level of a white pixel (having the minimum density level)is 0 and the gradation level of a black pixel (having the maximumdensity level) is 255. More specifically, the gradation level of aground portion is calculated using the following equation.Ground Portion Gradation Level=255−Image Portion Gradation Level

The edge determination unit 2103 detects a boundary between an imageportion and a ground portion in a target pixel update direction. Theground edge determination unit 2113 detects a boundary between a groundportion and an image portion in a target pixel update direction. Thecorrection amount storage unit 2105 stores a gradation correction amountcorresponding to a cumulative gradation value and a non-image cumulativegradation value. The correction unit 2104 corrects the gradation valueof a target pixel, which has been determined to be an edge by the edgedetermination unit 2103 and the ground edge determination unit 2113,with a correction amount corresponding to the edge stored in thecorrection amount storage unit 210. The corrected image data is outputto the pulse width modulation processor 22.

[Line Width Correction Process]

FIG. 8 is a flowchart illustrating a process performed by the line widthcorrection processor 21 according to the second embodiment. When animage formation start instruction is made, the CPU 201 transmitsline-by-line image data output from a printer driver to the image datastorage unit 2101. In step S201, the CPU 201 reads out image data fromthe RAM 203. The CPU 201 performs line width correction processing uponthe read image data of a processing line while updating a target pixel.

FIG. 9A is a diagram illustrating exemplary image data including hollowlines with different widths (L12=3-pixel width, L22=2-pixel width,L23=1-pixel width, and L24=0.5-pixel width) output by an imageprocessing apparatus.

Referring to FIG. 9B, a reference numeral 901 represents a series ofpixels in a horizontal line L illustrated in FIG. 9A as exemplary imagedata of a processing line read in step S201. In the second embodiment,line width correction processing is performed while sequentiallyupdating a target pixel from a pixel P1 to a pixel P17. A numericalvalue in each pixel represents a white gradation level.

In step S202, the cumulative gradation value acquisition unit 2102calculates a cumulative gradation value that is the sum of whitegradation levels of pixels in a target pixel update direction. A dashedframe 902 in FIG. 9B represents a cumulative gradation value calculatedand stored by the cumulative gradation value acquisition unit when eachpixel (P1 to P17) becomes a target pixel. For example, when a targetpixel is P3, P4, P5, P8, P9, P12, or P15 having a gradation valuegreater than or equal to 1, the gradation value of the target pixel isadded to a cumulative gradation value at a preceding pixel to calculatea cumulative gradation value at the target pixel. On the other hand,when a target pixel is P1, P2, P6, P7, P10, P11, P13, P14, P16, or P17having a gradation value of 0, a cumulative gradation value at thetarget pixel is reset to 0.

In step S203, the ground cumulative gradation value acquisition unit2112 calculates a ground cumulative gradation value that is thecumulative sum of black gradation levels of pixels in a target pixelupdate direction. A dashed frame 903 in FIG. 9B represents a groundcumulative gradation value calculated and stored by the groundcumulative gradation value acquisition unit 2112 when each pixel (P1 toP17) becomes a target pixel. In the second embodiment, in a case wherethe gradation value of a target pixel is less than or equal to 254, adifference between the maximum gradation value (255) and the gradationvalue of the target pixel (a reversal gradation value) is calculated asa ground cumulative gradation value. In a case where the gradation valueof a target pixel is 255, a non-image cumulative gradation value isreset to 0. For example, in a case where a target pixel is P1, P2, P6,P7, P10, P11, P13, P14, P16, or P17 having a gradation value less thanor equal to 254, the reversal gradation value of the target pixel isadded to a cumulative gradation value at the preceding pixel tocalculate a non-image cumulative gradation value at the target pixel. Onthe other hand, in a case where a target pixel is P3, P4, P5, P8, P9,P12, or P15 having a gradation value of 255, a non-image cumulativegradation value is reset to 0.

In step S204, the edge determination unit 2103 determines whether thetarget pixel is an edge of a white image. A dashed frame 904 in FIG. 9Brepresents whether the edge determination unit has determined that eachpixel (P1 to P17) is an edge when becoming a target pixel (edge: 1 andnon-edge: 0). In the second embodiment, when a cumulative gradationvalue at a target pixel is greater than or equal to 1 and the gradationvalue of a pixel adjacent to the target pixel (hereinafter referred toas an adjacent pixel) is 0, the target pixel is determined to be animage edge. In this embodiment, the pixels P5, P9, P12, and P15 aredetermined to be an image edge. In a case where the target pixel is animage edge (S204: YES), the process proceeds to step S205. On the otherhand, in a case where a target pixel is not an image edge (S204: NO),the process proceeds to step S206.

In step S205, the correction unit 2104 refers to a correction amounttable in the correction amount storage unit to acquire a correctionamount based on the cumulative gradation value. FIG. 10A is a diagramillustrating an example of a correction amount table stored in thecorrection amount storage unit. This table stores gradation correctionamounts corresponding to the cumulative gradation values of 1 to 1024.Referring back to FIG. 9B, a dashed frame 906 represents the amount ofcorrection acquired from the correction amount table in the correctionamount storage unit on the basis of a cumulative gradation value at apixel determined to be an edge.

In step S206, the ground edge determination unit 2113 determines whetherthe target pixel is an edge of a black image. A dashed frame 905 in FIG.9B represents whether the ground edge determination unit 2113 hasdetermined that each pixel (P1 to P17) is a ground edge when becoming atarget pixel (edge: 1 and non-edge: 0). In the second embodiment, when aground cumulative gradation value at a target pixel is greater than orequal to 1 and the gradation value of pixel adjacent to the target pixel(hereinafter referred to as an adjacent pixel) is 255, the target pixelis determined to be an non-image edge. In this embodiment, the pixelsP2, P7, and P11 are determined to be a non image edge. In a case wherethe target pixel is a ground edge (S206: YES), the process proceeds tostep S207. On the other hand, in a case where the target pixel is not aground edge (S206: NO), the process proceeds to step S209.

In step S207, the correction unit 2104 refers to a correction amounttable in the correction amount storage unit 2105 to acquire the amountof ground width correction based on the ground cumulative gradationvalue. FIG. 10B is a diagram illustrating an example of a correctionamount table stored in the correction amount storage unit 2105. Thistable stores gradation correction amounts corresponding to the groundcumulative gradation values of 1 to 1024. The larger the cumulativegradation value, the small the absolute value of a correction amount.

Referring back to FIG. 9B, a dashed frame 907 represents the amount ofcorrection acquired from the correction amount table in the correctionamount storage unit on the basis of a ground cumulative gradation valueat a pixel determined to be a ground edge.

In step S208, the correction unit 2104 adds the amount of gradationcorrection acquired in steps S205 and S207 to the target pixel. In acase where the gradation value of the target pixel acquired after theaddition of the amount of gradation correction in step S208 is greaterthan the maximum gradation level of 255 that can be expressed at thetarget pixel, the excess gradation values are distributed to an adjacentpixel. For example, when a target pixel is P5, P9, or P12 having thegradation value of 255, the gradation value of the target pixel afterthe addition of the amount of correction exceeds the maximum gradationlevel that can be expressed at the target pixel. Accordingly, the amountof correction is added to the gradation value of the adjacent pixel P6,P10, or P13. When a target pixel is P15, the value of: +127 out of thecorrection amount of +192 is added to the target pixel to achieve themaximum density level of 255 at the target pixel and then the remainingcorrection amount (192−127=65) is added to the adjacent pixel P16.

In the case of an engine that expresses a white pixel using a light beamOFF signal, in order to couple a light beam OFF signal at an adjacentpixel to a light beam OFF signal at a target pixel (in order to shift alight beam ON signal rearward), shift control information at theadjacent pixel is rewritten.

In a case where the gradation value of a target pixel acquired after theaddition of the amount of gradation correction is less than the minimumgradation level of 0 that can be expressed at the target pixel, thenegative gradation values are distributed to an adjacent pixel. Forexample, when the target pixel is P2, P7, or P11 having the gradationvalue of 0, the gradation value of the target pixel after the additionof the amount of correction is less than the minimum gradation levelthat can be expressed at the target pixel. Accordingly, the amount ofcorrection is added to the gradation value of the adjacent pixel P3, P8,or P12.

In the case of an engine that expresses a black pixel using a light beamON signal, in order to couple a light beam ON signal at an adjacentpixel to a light beam ON signal at a target pixel (in order to shift alight beam ON signal forward), shift control information at the adjacentpixel is rewritten.

In the case of target pixel upon which both the correction of an imageportion and the correction of a ground portion are performed at the sametime, the difference between the amount of correction of an imageportion and the amount of correction of a ground portion (| imageportion correction amount|−|ground portion correction amount|) may becalculated and set as the amount of correction for the target pixel.When a target pixel is P12, the sum of the image portion correctionamount of +128 at the pixel P12 in the dashed frame 906 and thecorrection amount of −16 in the dashed frame 907 which is distributedfrom the pixel P11 to the pixel P12 is calculated and a result of thesummation (+102) is added to the target pixel. At that time, since thegradation value of the target pixel is 255 and the gradation value ofthe target pixel after the addition of the correction amount exceeds themaximum gradation level that can be expressed at the target pixel, thecorrection amount of 102 is added to the gradation value of the adjacentpixel P13.

In step S209, the target pixel is shifted to the next pixel, forexample, in the right-hand direction by one pixel. In step S210, it isdetermined whether all pixels included in the image data acquired instep S201 have already been processed. In a case where it is determinedthat all pixels have yet to be processed, the process from step S202 tostep S210 is repeated. In a case where it is determined that all pixelshave already been processed, the process ends. Referring to FIG. 9B,image data 908 represents a result of pieces of processing of steps S202to S210 performed upon the image data 901. As represented in the imagedata 908, in the line width correction processing according to thesecond embodiment, the gradation value of a target pixel determined tobe an image edge or a ground edge is corrected using the amount ofcorrection based on the line width of an image portion or a groundportion at each pixel in a target pixel update direction. Accordingly,not only an image portion but also a ground portion can be preventedfrom undergoing thinning or a detail loss.

In the second embodiment, the enlargement of an image portion isperformed at the rear end of the image portion and the enlargement of aground portion is performed at the front end of an image portion (therear end of the ground portion). Since the image portion enlargement andthe ground portion enlargement do not interfere with each other, linewidth correction can be appropriately performed.

Third Embodiment

In the first embodiment, correction processing is performed on the basisof the gradation value of each pixel in image data. In the thirdembodiment, a single pixel includes a plurality of pixel pieces, andcorrection processing is performed on the basis of the number of pixelpieces representing an ON or OFF state. In the third embodiment, thesame reference numerals are used to identify configurations alreadydescribed in the first embodiment, and the description thereof will betherefore omitted.

In the third embodiment, the gradation level of each pixel in image dataranges from 0 to 16 (17 gradation levels). It is assumed that a printerdriver in the image processing apparatus 1 outputs image data having 17gradation levels. Alternatively, the image forming apparatus 2 may havea configuration for converting image data having 256 gradation levelsinto image data having 17 gradation levels. In the third embodiment, asingle pixel includes 16 pixel pieces, and pixel pieces, the number ofwhich corresponds to a gradation value, are set to ON state (binaryexpression). In this embodiment, an exposed portion becomes white.Accordingly, in each pixel, white pixel pieces, the number of whichcorresponds to a gradation value, are present. The larger the number ofwhite pixels in a pixel, the longer an exposure period for the recordingof the pixel. The number of pixel pieces included in each pixel is setto 16 in the third embodiment, but does not necessarily have to be 16.By providing N number of pixel pieces in a single pixel, (N+1) gradationlevels can be expressed at the pixel.

<Details of Line Width Correction Processing>

Line width correction processing will be described in detail below. FIG.11 is a block diagram illustrating functions performed by the CPU 201according to the third embodiment. The line width correction processor21 is a mechanism for performing line width correction processing uponimage data transmitted from a printer driver, and includes the imagedata storage unit 2101, the cumulative gradation value acquisition unit2102, the edge determination unit 2103, the correction unit 2104, andthe correction amount storage unit 2105.

The CPU 201 acquires image data (for example, 8-bit image data) in ascanning direction of light beam output from an image forming apparatusin units of lines and stores them in the image data storage unit 2101 inthe RAM 203. A single pixel includes a plurality of pixel pieces forcontrolling the ON/OFF states of light. FIG. 12 is a diagramillustrating an exemplary pixel. Referring to the drawing, a singlepixel includes 16 pixel pieces whose state can be switched between animage portion and a around portion. That is, at a single pixel, 16gradation levels can be expressed. The size of a single pixel in thehorizontal direction is equivalent to 1 dot (the width of 42.3 μm) at600 dpi. The size of a single pixel piece in a single pixel is thereforeequivalent to 1/16 dot (the width of approximately 2.65 μm).

The cumulative gradation value acquisition unit 2102 calculates andstores a cumulative gradation value that is the cumulative sum of pixelpieces in each pixel in image data. In the third embodiment, in order toimprove the reproducibility of a hollow line where thinning or a detailloss easily occurs because of degradation in an electrostatic latentimage, line width correction processing is performed focusing on thegradation level of a white portion. The cumulative gradation valueacquisition unit 2102 therefore calculates the cumulative such ofcontinuous white pixel pieces.

The edge determination unit 2103 detects the boundary between an imageportion and a ground portion in image data. The correction amountstorage unit 2105 stores the amount of gradation correction based on acumulative gradation value. The correction unit 2104 refers to theamount of correction stored in the correction amount storage unit 2105and corrects the gradation value of a target pixel determined to be anedge by the edge determination unit 2103. The corrected image data isoutput to the driving unit 204.

<Exemplary Line Width Correction Processing>

Referring to FIG. 13, a reference numeral 1301 represents an example ofimage data of a processing line. In the third embodiment, line widthcorrection processing is performed while sequentially updating a targetpixel from a pixel P1 to a pixel P6.

The cumulative gradation value acquisition unit 2102 calculates acumulative gradation value that is the cumulative sum of white pixelpieces (ON-state pixel pieces) in each pixel in a target pixel updatedirection. A dashed frame 1302 in FIG. 13 represents a cumulativegradation value calculated and stored by the cumulative gradation valueacquisition unit 2102 when each pixel (P1 to P6) becomes a target pixel.In the third embodiment, in a case where a white pixel piece in a targetpixel is adjacent to a preceding pixel (left-hand pixel), the number ofwhite pixel pieces in the target pixel is added to a cumulativegradation value. In a case where a white pixel piece in a target pixelis not adjacent to a preceding pixel, a cumulative gradation value isreset to 0 and the number of white pixel pieces is then added to thecumulative gradation value. For example, when a target pixel is P1, awhite pixel piece is not adjacent to a preceding pixel. A cumulativegradation value is therefore reset to 0 and then the number of whitepixel pieces is added to the cumulative gradation value. In thisexample, since the number of white pixel pieces is 0, a cumulativegradation value becomes 0. When a target pixel is P2, a white pixelpiece is not similarly adjacent to a preceding pixel. A cumulativegradation value is therefore reset to 0 and then the number of whitepixel pieces is added to the cumulative gradation value. In thisexample, since the number of white pixel pieces is 12, the cumulativegradation value becomes 12. When a target pixel is P3, a white pixelpiece is adjacent to a preceding pixel. The number of white pixel piecesis therefore added to a cumulative gradation value. In this example,since the number of white pixel pieces is 4, a cumulative gradationvalue becomes 16. Thus, at pixels P3 to P6, processing is similarlyperformed. In this embodiment, even in a case where a cumulativegradation value is greater than or equal to 64, a signal value of 64 isoutput to limit the number of bits of a signal. The reason for this isthat it is unnecessary to perform line width correction processing uponan image including a heavy line including 64 continuous pixel pieces ormore that are image portions.

The edge determination unit 2103 determines whether the target pixel islocated at the boundary between an image portion and a ground portion. Adashed frame 1303 in FIG. 13 represents whether the edge determinationunit has determined that each pixel (P1 to P6) is an edge when becominga target pixel (edge: 1 and non-edge: 0). In the third embodiment, in acase where a target pixel includes a white pixel piece whose rear end isadjacent to a black pixel piece, the target pixel is determined to be anedge. For example, when a target pixel is P1, P2, P4, or P6, the targetpixel is determined to be a non-edge (0) because the target pixel doesnot include a white pixel piece whose rear end is adjacent to a blackpixel piece. On the other hand, when a target pixel is P3 or P5, thetarget pixel is determined to be an edge (1) because the target pixelincludes a white pixel piece whose rear end is adjacent to a black pixelpiece. In the case of a target pixel such as P2 or P5 including a whitepixel piece adjacent to an adjacent pixel, the determination of whetherthe target pixel is an edge can be performed by referring to the firstpixel piece in the adjacent pixel.

In a case where it is determined that the target pixel is an edge, theCPU 201 refers to the correction amount table in the correction amountstorage unit to acquire a correction amount based on the cumulativegradation value. FIG. 14 is a diagram illustrating an example of acorrection amount table stored in the correction amount storage unit.This table stores gradation correction amounts corresponding to thecumulative gradation values of 1 to 64. Referring back to FIG. 13, adashed frame 1304 represents the amount of correction acquired from acorrection amount table in the correction amount storage unit on thebasis of a cumulative gradation value at a pixel determined to be anedge.

The CPU 201 changes a black pixel piece adjacent to a white pixel pieceto a white pixel piece on The basis of the amount of correction based onthe cumulative gradation value at the target pixel to increase the widthof an image portion (white pixel). In a case where the amount ofcorrection exceeds the number of black pixel pieces in the target pixel,black pixel pieces in the adjacent pixel are sequentially changed to awhite pixel piece starting from the first pixel piece.

Image data 1305 in FIG. 13 represents a result of line width correctionprocessing according to the third embodiment performed upon the imagedata 1301. The line width of image data is acquired by calculating thecumulative sum of pixel pieces in each pixel in a target pixel updatedirection, and a pixel piece located at an edge is corrected using theamount of correction based on the line width. As a result, it ispossible to appropriately improve the reproducibility of an output imagewith a small reference data range and the small amount of computation.

Other Embodiments

In the above-described embodiments, toner is not applied to an exposedportion in an image forming apparatus. However, an image formingapparatus in which toner is applied to an exposed portion may be used.

Embodiment (s) of the present invention can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2015-230697 filed Nov. 26, 2015, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image forming apparatus for forming an imageby performing scanning exposure upon a photosensitive member on thebasis of a gradation value represented by pixel data, comprising: areading unit configured to sequentially read, from image data, pieces ofpixel data of target pixels along a predetermined direction; acumulative sum calculation unit configured to calculate a cumulative sumof gradation values represented by pieces of pixel data of target pixelsin a significant image portico read by the reading unit; a determinationunit configured to determine whether the target pixel read by thereading unit is located at a boundary between the significant imageportion and a background portion that is a non-significant imageportion; and a correction unit configured to, in a case where thedetermination unit determines that the target pixel is located at theboundary, correct a gradation value represented by pixel data of thetarget pixel by adding a correction amount based on the cumulative sumup to the target pixel calculated by the cumulative sum calculation unitto the gradation value.
 2. The image forming apparatus according toclaim 1, wherein, in a case where a gradation value represented by pixeldata of a target pixel is a value representing a background portion, thecumulative sum calculation unit resets the cumulative sum.
 3. The imageforming apparatus according to claim 1, wherein, in a case where agradation value represented by pixel data of the target pixel includes asignificant image component and a gradation value represented by pixeldata of a next pixel does not include a significant image component, thedetermination unit determines that the target pixel is located at anedge.
 4. The image forming apparatus according to claim 1, wherein thesmaller the cumulative sum, the larger a correction amount that thecorrection unit adds to a gradation value represented by pixel data ofthe target pixel.
 5. The image forming apparatus according to claim 1,further comprising a pulse-width modulation (PWM) unit configured tocontrol an exposure period for a single pixel by determining the numberof pixel pieces to be subjected to exposure out of N number of pixelpieces set in advance in the pixel along a direction of the scanningexposure.
 6. The image forming apparatus according to claim 1, furthercomprising: a second cumulative sum calculation unit configured tocalculate a cumulative sum of differences between a gradation valuerepresented by pixel data of a target pixel read by the reading unit andan upper limit of a gradation value of a significant image portion as acumulative sum of gradation values of non-significant image portions; asecond determination unit configured to determine whether the targetpixel read by the reading unit is located at a boundary between thenon-significant image portion and the significant image portion; and asecond correction unit configured to, in a case where the seconddetermination unit determines that the target pixel is located at theboundary, correct a gradation value represented by pixel data of thetarget pixel by adding a correction amount based on the cumulative sumup to the target pixel calculated by the second cumulative sumcalculation unit to the gradation value.
 7. A non-transitory storagemedium storing a program causing a computer to execute a control methodfor an image forming apparatus for forming an image by performingscanning exposure upon a photosensitive member on the basis of agradation value represented by pixel data, the method comprising:sequentially reading, from image data, pieces of pixel data along apredetermined direction; calculating a cumulative sum of gradationvalues represented by pieces of pixel data of target pixels in asignificant image portion read in the reading; determining whether thetarget pixel read in the reading is located at a boundary between thesignificant image portion and a background portion that is anon-significant image portion; and correcting, in a case where it isdetermined that the target pixel is located at the boundary in thedetermining, a gradation value represented by pixel data of the targetpixel by adding a correction amount based on the cumulative sum up tothe target pixel calculated in the calculating to the gradation value.8. A control method for an image forming apparatus for forming an imageby performing scanning exposure upon a photosensitive member on thebasis of a gradation value represented by pixel data, comprising:sequentially reading, from image data, pieces of pixel data along apredetermined direction; calculating a cumulative sum of gradationvalues represented by pieces of pixel data of target pixels in asignificant image portion read in the reading; determining whether thetarget pixel read in the reading is located at a boundary between thesignificant image portion and a background portion that is anon-significant image portion; and correcting, in a case where it isdetermined that the target pixel is located at the boundary in thedetermining, a gradation value represented by pixel data of the targetpixel by adding a correction amount based on the cumulative sum up tothe target pixel calculated in the calculating to the gradation value.